Department of Radiology, University Hospital (CHUV) and University of Lausanne (UNIL), Lausanne, Switzerland.
Lausanne and Geneva, Switzerland.
NMR Biomed. 2021 Jan;34(1):e4418. doi: 10.1002/nbm.4418. Epub 2020 Oct 1.
Fluorine-19 ( F) MRI of injected perfluorocarbon emulsions (PFCs) allows for the non-invasive quantification of inflammation and cell tracking, but suffers from a low signal-to-noise ratio and extended scan time. To address this limitation, we tested the hypotheses that a F MRI pulse sequence that combines a specific undersampling regime with signal averaging has both increased sensitivity and robustness against motion artifacts compared with a non-averaged fully sampled pulse sequence, when both datasets are reconstructed with compressed sensing. As a proof of principle, numerical simulations and phantom experiments were performed on selected variable ranges to characterize the point spread function of undersampling patterns, as well as the vulnerability to noise of undersampling and reconstruction parameters with paired numbers of x signal averages and acceleration factor x (NAx-AFx). The numerical simulations demonstrated that a probability density function that uses 25% of the samples to fully sample the k-space central area allowed for an optimal balance between limited blurring and artifact incoherence. At all investigated noise levels, the Dice similarity coefficient (DSC) strongly depended on the regularization parameters and acceleration factor. In phantoms, the motion robustness of an NA8-AF8 undersampling pattern versus NA1-AF1 was evaluated with simulated and real motion patterns. Differences were assessed with the DSC, which was consistently higher for the NA8-AF8 compared with the NA1-AF1 strategy, for both simulated and real cyclic motion patterns (P < 0.001). Both strategies were validated in vivo in mice (n = 2) injected with perfluoropolyether. Here, the images displayed a sharper delineation of the liver with the NA8-AF8 strategy than with the NA1-AF1 strategy. In conclusion, we validated the hypotheses that in F MRI the combination of undersampling and averaging improves both the sensitivity and the robustness against motion artifacts.
氟-19（F）MRI 对注入的全氟碳乳液（PFC）进行非侵入性定量炎症和细胞示踪，但存在信噪比低和扫描时间长的问题。为了解决这个限制，我们测试了以下假设：与未平均的完全采样脉冲序列相比，结合特定欠采样方案和信号平均的 F MRI 脉冲序列在使用压缩感知重建时，无论是在具有配对 x 信号平均和加速因子 x（NAx-AFx）的欠采样和重建参数的噪声下，都具有更高的灵敏度和对运动伪影的鲁棒性。作为原理验证，在选定的变量范围内进行了数值模拟和体模实验，以表征欠采样模式的点扩散函数，以及欠采样和重建参数的噪声脆弱性。数值模拟表明，使用 25%的样本对 k 空间中心区域进行全采样的概率密度函数允许在有限的模糊和伪影不连贯性之间实现最佳平衡。在所有研究的噪声水平下，Dice 相似系数（DSC）强烈依赖于正则化参数和加速因子。在体模中，使用模拟和真实运动模式评估了 NA8-AF8 欠采样模式与 NA1-AF1 之间的运动稳健性。使用 DSC 评估了差异，对于模拟和真实循环运动模式，NA8-AF8 与 NA1-AF1 策略相比，DSC 始终更高（P <0.001）。这两种策略都在注射全氟聚醚的小鼠体内进行了验证（n=2）。在这里，与 NA1-AF1 策略相比，NA8-AF8 策略显示出更清晰的肝脏轮廓。总之，我们验证了以下假设：在 F MRI 中，欠采样和平均的结合提高了灵敏度和对运动伪影的鲁棒性。
